The present invention relates to the field of electrical resistance welding machines and concerns a rolling head for making seam welds on metal plates, especially thin metal plates which, starting from an initial flat state, are first curved in such a way that their two ends are brought edge to edge or slightly overlapped and then joined to form tubular elements or pipes used for a wide variety of different applications. The main application for such tubular elements is the manufacture of cans for foodstuffs.
A welding head of this type (especially one that constitutes the welding portion that is positioned inside the tubular element to be welded) essentially comprises two separate component parts. One of the components is a cylindrical shaft that is stationarily and rigidly mounted on a supporting arm and acts as a stator member of the assembly. The other is a ring-shaped component mounted coaxially on the stator member and rolls on the parts to be joined, thus acting as a rolling electrode (rotor member). When electrical current is passed through it, it works in conjunction with a matching, rotary welding disc located outside the tin can to make a seam weld.
The conduction of electrical current through the head between the fixed stator and the mobile rotor is accomplished by an electrical conductive device located between these two parts and electrically connected to them.
Electrical conductive devices of this kind used in the past (for example, see U.S. Pat. No. 4,188,523) used the conductive properties of a fluid in the liquid state: for example, mercury or eutectic mixes of other chemical substances.
In conductive devices of more recent design, electrical current is passed between the stator and the rotor through a mechanical, sliding contact.
A welding head of this type, disclosed in publication EP 0459 091 and made according to the preamble to independent claim herein, essentially comprises: a stator; an annular electrode constituting the rotor, rotatably mounted on the stator and coaxial therewith; a device for conducting electricity through a sliding contact, positioned between the stator and the rotor and pressed in axial direction against at least one of them by an elastic force; and conduits through which a coolant flows in the stator, in the rotor and in the sliding contact conductive device.
Heads of this kind have proved fully satisfactory for practical purposes. However, they are still subject to considerable improvements forming the subject-matter of the present invention.
Accordingly, the invention has for a first object to provide a welding head which allows higher currents to be exchanged between stator and rotor than prior art welding heads without increasing the size of the welding head compared to prior art heads of the same kind.
A further object of the invention is to more effectively dissipate heat, which, combined with the higher welding current, increases both the welding performance and the useful life of the welding head according to the invention, with obvious economic advantages.
Another object of the invention is to provide a welding head where the electrical current passes between stator and rotor without breaks, even at high production speeds and in the presence of strong vibrations and/or local irregularities in the materials to be welded.
Yet another object of the invention is to provide a welding head of simpler construction than prior art heads and that is quicker and easier to assemble and maintain.
According to the invention, these and other objects are achieved by a rolling head for a resistance seam welding machine comprising: a stator; an annular electrode constituting the rotor, rotatably mounted on the stator and coaxial therewith; a device for conducting electricity through a sliding contact, positioned between the stator and the rotor and pressed in axial direction against at least one of them by an elastic force; and conduits through which a coolant flows in the stator, in the rotor and in the sliding contact conductive device; wherein the sliding contact conductive device comprises at least one elastically yielding element forming a radial sector, positioned between the stator and the rotor and designed to remain permanently in contact with the stator and rotor so as to conduct electrical current from one to the other.